This invention relates to a television horizontal automatic frequency and phase control (AFPC) loop in which the phase detector compares horizontal synchronizing signals with oscillator signals at a frequency twice that of the horizontal deflection drive for maintaining AFPC gain during the vertical synchronizing and equalizing pulse intervals.
Television displays of broadcast television signals are generated by repetitively scanning an electron beam over the surface of a picture tube viewing screen to form a lighted raster area. The beam intensity is modulated by video signals to form images on the screen representative of the picture to be displayed. Conventional television provides a high-speed horizontal scanning in conjunction with a relatively low-speed vertical scanning. The scanning in the vertical and horizontal directions is synchronized with synchronizing signals included in a composite video signal with the video signal to be displayed. The synchronizing signals are extracted from the composite video, and the synchronizing signals thus extracted are used to synchronize the vertical and horizontal-direction scanning apparatus.
The synchronizing signals are extracted from the composite video by use of synchronizing signal separator circuits. A sync separator for separating the horizontal synchronizing signal from the composite video includes a differentiating circuit and a threshold circuit. The differentiating circuit selectively couples signals at and above the horizontal synchronizing frequency to the threshold circuit. The threshold circuit responds to the differentiated synchronizing-signal portions of the composite video to produce a sequence of constant-width pulses representing horizontal frequency, synchronizing portions of the composite video.
The vertical synchronizing signals contained in the composite video are high-amplitude pulses having low-frequency components. The vertical synchronizing signal proper has a duration of three horizontal lines. In order to maintain the flow of horizontal synchronizing information during the vertical synchronizing interval, the vertical synchronizing pulse includes serrations by which the horizontal oscillator may be synchronized. In the NTSC television system, vertical scanning of an image is accomplished during two successive field intervals, the horizontal scanning lines of which are interlaced. Interlacing requires that the vertical oscillator timing be maintained in an exact relationship with the horizontal frequency. In order to help the vertical sync detector to maintain exact timing in extracting the vertical synchronizing pulses, equalizing pulses are provided in the composite video during a period of three horizontal lines preceding and following the vertical synchronizing intervals. The equalizing pulses recur at twice the rate of the horizontal synchronizing pulses.
In television systems in which the composite video signals are modulated onto a carrier and broadcast, many of the television receivers are in areas far from the transmitting station, where a weak signal can be expected. Due to the presence of unavoidable thermal noise, and also due to various forms of interference signals which may occur in the vicinity of the receiver, it may be expected that the composite video as received and the synchronizing signals derived therefrom will be intermingled with electrical noise. This electrical noise is manifested as random variation of the desired signal amplitude, and can severely perturb the operation of the display device. Commonly, noisy synchronizing causes "rolling" or "tearing" of the image displayed on the raster. As transmitted, the synchronizing signal pulses recur at a rate which is carefully controlled and extremely stable. Since the presence of noise obscures the synchronizing signals in a random manner, it has become common practice to obtain synchronization of the horizontal deflection circuit with the horizontal synchronizing pulse signal by the use of an oscillator, the free-running frequency of which is near the horizontal scanning frequency, and the exact frequency and phase of which is controlled in an indirect manner by a phase-lock loop (PLL) to equal the synchronizing signal frequency and phase. Thus, when any one synchronizing pulse is obscured by noise, the rate of the oscillator remains substantially unchanged, and the deflection circuits continue to receive regular deflection control pulses.
In a PLL, a phase detector compares the output of the horizontal oscillator with the horizontal synchronizing pulses from the sync separator and produces a pulsating control signal representative of the frequency and phase difference between the two. The control signal is then filtered and applied to the oscillator in such a manner as to maintain the oscillator in frequency and phase synchronism with the average frequency and phase of the received synchronizing pulses.
Since the PLL is a feedback system, there is an undesirable residual phase error between the oscillator signal and the synchronizing signal. High loop gain is desirable in order to minimize error, but, the loop then becomes more responsive to perturbing noise. This can be offset by reducing the closed-loop bandwidth of the PLL, which may undesirably reduce transient response time. Thus, a compromise between loop gain and bandwidth is often necessary.
With the advent of integrated circuits for low-power signal processing in television devices, it has become convenient in a PLL to compare the horizontal synchronizing signals from the sync separator with a square wave as produced by the controlled horizontal oscillator rather than with a sawtooth signal. During the synchronizing pulse interval, the PLL phase detector gates a first current source which charges a storage capacitor in a first polarity and when the oscillator square wave output is high, and which turns off the first current source and turns on a second current source poled to discharge the capacitor when the oscillator output is low. Thus, when the transition time of the square-wave oscillator output is centered on the synchronizing pulse, the charging and discharging effects are equal and the net capacitor voltage does not change. This maintains the oscillator frequency constant.
With the described type of phase detector, the phase detector gain and therefore the loop gain of the PLL may decrease during the equalizing and vertical synchronizing pulse intervals. The phase detector gain drops because during the vertical sychronizing and equalizing pulse intervals the sync signal occurs twice during each VCO output square wave, and thus the phase detector compares during both the rise and fall times of the square wave. Changes in oscillator phase which change the phase detector output during one half of the square wave result in an equal and opposite change during the other half of the square wave and no net change in output results. Thus the oscillator may drift in an uncontrolled manner during the vertical synchronizing and equalizing pulse intervals.
Such a decrease in gain of the PLL may be disadvantageous when rapid slewing of the horizontal oscillator frequency or phase is required during the vertical blanking interval. This may be the case, for example, when the television receiver is to be used to display information which has been recorded on a home-type video tape recorder. Such tape recorders often have a plurality of reproduction heads, each of which is mechanically scanned across the tape. In one common scheme, two heads are used, which alternately scan the tape for a duration equal to that of a vertical field. In order to avoid loss of, or breaks in, the displayed information, scanning of the succeeding field is commenced by the second head substantially concurrently with the end of scanning in the first head. However, slight differences in tape tension or in the dimensions of the mechanical tape transport acting on the tape for playback compared with the tension and dimensions when the tape was recorded results in differences in the time between succeeding horizontal synchronizing pulses in the information as recorded as compared with playback, especially during the switchover between heads. This results in a discontinuity or step change in the phase of the horizontal synchronizing pulses available for synchronizing the horizontal oscillator, which step normally occurs about five horizontal lines before the end of a vertical scanning interval and the beginning of the vertical blanking interval. A high oscillator slew rate during the vertical blanking interval is necessary to conform the horizontal oscillator phase to the synchronizing signal phase after the step change, and this conformance must be complete before scanning begins for the next succeeding field in order to correctly reproduce the desired image. A decrease in PLL gain during the equalizing and vertical synchronizing pulse intervals as may be occasioned by the presence of equalizing pulses may prevent rapid slewing of the horizontal oscillator and therefore prevent accomodation of such a step change. This may result in an apparent bending or tearing of vertical lines in the displayed image at the top of the raster.